when brake portal block (NOT obsidian) not all the portal gets destroyd, only that block

when you break a portal block so not all rthe portal breaks. i tried making a portal on/off system turned on, put tnt and only a few blocks broke then when to a test world and made a portal and brock in creative 1 of the portal blocks(the spining thing) and only that one brock if i want through that one i didnt go to nether but any other part of the portal brang me into the nether.